The invention relates to an apparatus and method for cryogenically cooling a cutting tool. More particularly, the invention relates to a cutting tool which provides cryogenic cooling to the point of the tool which experiences the highest temperature during cutting and a method thereof. Furthermore, the invention is drawn to a chip breaker which contains a channel for cooling the tool.
Metal cutting or machining is a chip forming process. When the cutting tool engages a rotating workpiece with a fixed feed, a chip of material is formed by the tool penetrating into the workpiece. The chip formation involves two mechanisms referred to as primary deformation and secondary deformation. The process of primary deformation is actually a shearing process. The chip is fragmented from a ductile workpiece but rejoins itself, forming a continuous chip. Then friction between the chip and tool face deforms the bottom side of the chip, a process referred to as secondary deformation in the friction zone. Heat build up, as a result of this friction, increases the temperature of the tool drastically and concentrates the heat at the interface of the chip and the tool face. As the chip slides on the tool face, the temperature continues to increase until the contact pressure between the chip and tool drops. The highest temperature is experienced at a point along the tool face, also known as the rake face, but at a small distance from the cutting edge of the tool, as shown in the sample temperature distribution of FIG. 1 for normal conventional machining. The highest temperature point usually coincides with the area where crater wear occurs, a distance from the tool corner. This distance is approximately equal to the undeformed chip thickness or feed. Because of this high temperature, the cutting tool could overheat decreasing the tool life, increasing the force required to cut the surface and decreasing cutting accuracy.
When cutting ductile materials such as soft aluminum or low carbon steels, the heat causes a "stickiness" which produces material build-up on the edge of the cutting tool also, known as "welding." Conventionally, this problem has been solved by applying cutting fluid or cutting oil in a flood or mist directly to the back-side of the chips, the side opposite the cutting tool. This served as both a coolant and a lubricant. These cutting fluids however, resulted in a bad odor, water penetration into the machine bearings, and sanitary problems. In addition, they are prone to damaging the environment and may even cause health hazards.
The application of cryogenic fluid to cool the metal cutting process started as early as the 1950's. The cryogenic fluids used were CO.sub.2, freon, or solvenlene. They were sprayed in the general cutting area or were applied to the workpiece before cutting in a prechill. This method, however consumed excessive amounts of cryogenic fluid and had no lubrication effect. It has been demonstrated that it is generally undesirable to cool the workpiece by the cooling power of the liquid gas. In general, the strength of the workpiece material increases as the temperature drops, and the material becomes harder. Therefore, the tool requires more force to cut the harder material and the material becomes more difficult to machine.
Two known methods of using cryogenic fluids to cool the cutting tool are shown in patents to Philip and Dudley. As explained below, neither of these methods cool the face of the tool where the highest temperatures are encountered. Both cool the interior or backface of the tool.
U.S. Pat. No. 3,077,802 to T. B. Philip discloses carbon dioxide in the form of a liquid, gas or snow to cool the interior of a cutting tool. The carbon dioxide is allowed to expand into the vicinity of the cutting edge. A hole is provided in the middle of the cutting tool and a capillary is used to transport the carbon dioxide. Smaller holes are drilled to the tool surface for escape of gases. However, the cryogenic fluid does not reach the point of highest temperature in the tool and this has been proven to be unsatisfactory for the much higher cutting speeds used today.
U.S. Pat. No. 3,971,114 to G. M. Dudley discloses a cryogenic fluid routed through the tool which is discharged in a stream between the cutting edge and the workpiece when cutting. However, in Dudley, the cryogenic gases are not injected to the point of highest temperature and therefore, the effectiveness of the coolant is reduced. Secondly, in Dudley, carbon dioxide is injected at the interface between the cutting edge and the workpiece (i.e. at the backside of the tool). The coolant alone is used to cool the tool from the side surface, which is not the highest temperature area, thereby making the cooling comparatively ineffective. In addition, the workpiece cooling is not desirable and there is no lubrication effect provided by the methods as shown in Dudley or Philip.
With the improvement of machining technology, the cutting speed is generally much higher than in the past, the heat build up is greater, and the cutting temperature is so high that cooling by the indirect cooling methods of the prior art is no longer effective to cool the high temperatures. The method and apparatus of the present invention provide a practical solution for the improved machining of materials including high speed steel alloys, aluminum, titanium, low carbon steels, and composites.
Since the advent of carbide tools and the use of high cutting speeds, the continuous chips described above, produced in metal cutting have presented serious problems. At lower cutting speeds, these chips usually have a natural curl and tend to be brittle. However, cutting speeds have increased to such an extent that chip control is a necessity. In turning operations, where the tool is continuously removing metal for long periods, a continuous chip can become entangled with the tool, the workpiece, or the machine tool elements. This type of chip can be hazardous to the operator and, unless controlled properly, it can result in mechanical chipping of the cutting edge. In addition, the handling of long, continuous chips can present a major economic problem. For example, handling characteristic of chips can be expressed by their bulk ratio; namely, the total volume occupied by the chip divided by the volume of solid chip material. Unbroken, continuous chips have a bulk ratio of approximately 50. Tightly wound chips have a bulk ratio of approximately 15. Well-broken chips have a bulk ratio of approximately 3. The volume occupied by well-broken chips is, therefore, about one-seventeenth the volume of unbroken chips, a considerable advantage when it comes to handling and disposal.
When the chips are formed they have long grain structures, traverse to the direction of cutting. However, as the chip rubs the surface of the workpiece, the grain structure on the lower surface bends to form a "long tail" due to secondary deformation and the heat build up, as shown in FIG. 2. This lower surface then recrystallizes due to the grain structure and intense heat. This recrystallized area is highly undesirable since it is more difficult to have smaller, broken chips.
This invention has a special advantage in improving the chip breaking. The method of improving chip breaking for ductile material is through the following mechanisms: a) increasing the brittleness of the chip material by the low temperature application of the cryogenic coolant, b) reducing the secondary deformation that occurs at the bottom surface of the chip which rubs on the tool face, c) reducing the long tail in the grain structure of the chips, which is a major cause of the difficulty in breaking ductile chips, d) avoiding the welding or recrystallizing of the bottom layer of the chip (to avoid the strong bonding of chip structure which adds to the difficulty in chip breaking) by the coldness and reduction of the secondary deformation, and e) bending and curling the chip for breaking by the use of both a mechanical chip breaker and the pressure from the cryogenic fluid. This may occur when the liquid expands into gas within the nozzle.
Another problem encountered in conventional machining of soft, sticky, ductile materials, is the friction between the chip and the tool becomes so great that the chip material welds itself to the tool face. The presence of this welded material further increases the friction, and this friction leads to the build up of layer upon layer of chip material. The resulting pile of material is referred to as a build-up edge. The build-up edge often continues to grow and then breaks down when it becomes unstable, and the broken pieces are carried away by the underside of the chip and the new workpiece surface. The resulting workpiece surface is rough. Actually, build-up edge formation in metal cutting is the principal factor affecting surface finish and can have a considerable influence on premature failure of the cutting tool and cutting tool wear. This occurs because the build-up will accumulate on the tool face and pull the tool causing the tool to fall apart due to the two different thermal expansions of the two metals.
In this invention, however, the build-up edge formation is automatically prevented by injecting the super cold cryogenic fluid to the cutting tip area. This reduces the adhesion of the chip material to the tool tip which lowers the temperature in the cutting zone and prevents the welding phenomenon. Also, this invention can clean and remove the possible build-up edge with the cryogenic jet. Because the build-up edge is the main cause of poor surface quality, the method of the present invention also indirectly improves the finish of the machined surface. Therefore, it is an object of the present invention to provide a cutting tool which injects liquid nitrogen or other liquified gas as close as possible to the highest heat affected area of the cutting tool, not the workpiece, through a nozzle, between the chip and the tool face. This provides a fluid cushion which reduces the friction of the chip rubbing on the tool face, and reduces the cutting force involved, as well as the abrasive wear. Furthermore, a chip breaker is used to slightly lift up the chip so that coolant can penetrate to the highest temperature area and reduce the contact length of the chip on the tool face. This will improve the penetration of coolant to the cutting zone. Thus friction between the chip and the tool face is greatly reduced and therefore, heat due to the frictional deformation is greatly reduced.